Cytochrome C (horse heart: PDB 1HRC (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=233461034608315&page=0&pdbId=1HRC)) is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It is a soluble protein, unlike other cytochromes and is an essential component of the electron transfer chain. It is capable of undergoing oxidation and reduction, and does not bind oxygen. It transfers electrons between Complexes III and IV.

Cytochrome c is a highly conserved protein across the spectrum of species, found in plants, animals, and many unicellular organisms. This, along with its small size (molecular weight about 12,000 daltons), makes it useful in studies of evolutionary divergence.

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